1. Field of the Invention
The present invention to be disclosed in this specification relates to a semiconductor device capable of communicating (transmitting and receiving) data wirelessly (a data carrier). In particular, the present invention relates to a protection circuit which prevents breakage or deterioration of elements which are included in a semiconductor device when the semiconductor device receives high power in wireless communication.
2. Description of the Related Art
In recent years, individual identification technologies have attracted attention. In these technologies, an identification (ID) number is assigned to each object to recognize the history of information on the object and to be utilized in production, management, or the like. In particular, radio-frequency identification (RFID) techniques for transmitting and receiving data by wireless communication have been widely used. A wireless communication system using the RFID technique is a communication system including a wireless communication device (an interrogator) and a data carrier (a transponder), and data is wirelessly communicated between them. The wireless communication device refers to a device capable of wirelessly transmitting and receiving signals, such as a reader/writer, a cellular phone, or a personal computer. In this specification, the wireless communication device is typically referred to as a reader/writer. In addition, the data carrier is generally called an RFID, an RF tag, an ID tag, an IC tag, an IC chip, a wireless tag, an electronic tag, or the like. In this specification, the data carrier is typically referred to as an RFID.
Examples of RFIDs include passive RFIDs which do not have power sources and active RFIDs which incorporate power sources. The passive RFID has a function to convert a wireless signal (a carrier wave or an amplitude-modulated wave generated by superimposing a modulation wave on a carrier wave) transmitted from a reader/writer into a DC voltage by a rectifier circuit which is provided in the RFID, and circuits in the RFID operate using the DC voltage. The strength of the carrier wave is generally expressed as power, and power is attenuated as the distance between an emission point of the power and a measurement point of the power (hereinafter referred to as a communication distance in this specification) is increased. In other words, when the communication distance between the reader/writer and the RFID is changed, power which is supplied to the RFID is also changed in accordance with the change in communication distance. Then, in the case where the communication distance between the reader/writer and the RFID is extremely short (for example, in the case where they are in contact with each other), high power is supplied to the RFID. When the RFID is supplied with high power, the RFID might malfunction because of incorrect demodulation of signals from the reader/writer, or elements provided in the RFID might be deteriorated or damaged.
On the other hand, the wireless communication system using the RFID technique has an advantage that data in a plurality of RFIDs can be read at the same time by a reader/writer; however, when the data of the plurality of RFIDs is read at the same time, a communication distance between each of the plurality of RFIDs and the reader/writer might be different. Further, the communication distance changes from moment to moment in the case where a carton of products provided with an RFID is passed in front of a reader/writer by using a forklift. In view of the foregoing actual uses of such a wireless communication system, the communication distance might be extremely short unintentionally, and as a result, high power might be supplied to an RFID.
In order to resolve such a problem, provision of a protection circuit which protects elements in an RFID when high power is applied to the RFID has been known (for example, see Reference 1).
The protection circuit has a function to control the state of a dedicated path which allows high power to escape. When the communication distance is long and received power is low, the dedicated path which allows high power to escape is put in an off state so that the operation of the RFID is not adversely affected. On the other hand, when the communication distance is short and received power is high, the dedicated path which allows high power to escape is put in an on state so that high power is not applied to the elements in the RFID.